1. Field of the Invention
The invention relates to an optical disc and an optical disc reproducing apparatus in which a recording density is improved.
2. Description of the Prior Art
An optical disc such as a compact disc or the like is widely spread as a recording medium which can record a large quality of data. However, in the optical disc which is at present widespread, there is a limitation in a data recording capacity. It is presumed in future that a large amount of data such as image data or the like is recorded onto the optical disc. For preparation of such a case, the development of the optical disc which can realize a higher density and a larger capacity than those of the present optical disc is being progressed.
As a recording method of the optical disc, there are bit-by-bit method and a holographic method. According to the bit-by-bit method, a laser beam is converged to a small diameter and irradiated to pits one by one, thereby reproducing data one bit by one. Almost of the optical discs which are at present put into practical use use the bit-by-bit method. In the bit-by-bit method, a structure is simple and a process such as reproduction of clocks or the like can be easily performed. To raise a recording density in the bit-by-bit method, the pit diameter is reduced and the laser beam is converged until the pit of the small diameter can be reproduced.
A degree of the reduction of the diameter of the pit which is formed onto the optical disc depends on the advancement of the working technique. If the working technique is further progressed in future, the size of pit which can be formed on the optical disc can be further reduced than the present size. Since the spot diameter of the laser beam which can be converged, however, depends on a numerical aperture NA of the lens and a wavelength .lambda. of the laser beam, there is a limitation. According to the bit-by-bit method, therefore, the recording density is limited by the spot size of the laser beam and there is a limitation in the improvement of the recording density.
According to the holographic method, however, the recording density is not limited by the spot size of the laser beam. However, in the holographic method, it is difficult to reproduce the clocks and a reproduction signal is multi-dimensional and signal processes are difficult.